title: Critical differences between isoforms of securin reveal mechanisms of separase regulation.
abstract: Sister chromatid separation depends on the activity of separase, which in turn requires the proteolysis of its inhibitor, securin. It has been speculated that securin also supports the activation of separase. In this study, we found that PTTG1 was the major securin isoform expressed in most normal and cancer cell lines. Remarkably, a highly homologous isoform called PTTG2 was unable to interact with separase. Using chimeras between PTTG1 and PTTG2 and other approaches, we pinpointed a single amino acid that accounted for the loss of securin function in PTTG2. Mutation of the homologous position in PTTG1 (H(134)) switched PTTG1 from an inhibitor into an activator of separase. In agreement with this, PTTG1 lacking H(134) was able to trigger premature sister chromatid separation. Conversely, introduction of H(134) into PTTG2 is sufficient to allow it to bind separase. These data demonstrate that while the motif containing H(134) has a strong affinity for separase and is involved in inhibiting it, another domain(s) is involved in activating separase and has a weaker affinity for it. Although PTTG2 lacks securin function, its differences from PTTG1 provide evidence of independent inhibitory and activating functions of PTTG1 on separase.
authors.affiliation: Division of Life Science, Center for Cancer Research, and State Key Laboratory of Molecular Neuroscience, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong.
authors.forename: Xianxian
authors.initials: X
authors.lastname: Han
authors.forename: Randy Y C
authors.initials: RY
authors.lastname: Poon
mesh_terms: D000595:Amino Acid Sequence; D018797:Cell Cycle Proteins / Q000032:analysis / Q000235:genetics / Q000378:metabolism*; D010450:Endopeptidases / Q000032:analysis / Q000235:genetics / Q000378:metabolism*; D004789:Enzyme Activation; D006367:HeLa Cells; D056945:Hep G2 Cells; D006801:Humans; D008938:Mitosis; D008969:Molecular Sequence Data; D009363:Neoplasm Proteins / Q000032:analysis / Q000235:genetics / Q000378:metabolism*; D009369:Neoplasms / Q000235:genetics / Q000378:metabolism*; D017354:Point Mutation; D060066:Protein Interaction Maps; D020033:Protein Isoforms / Q000032:analysis / Q000235:genetics / Q000378:metabolism; D055550:Protein Stability; D017434:Protein Structure, Tertiary; D064249:Securin; D064247:Separase; D016415:Sequence Alignment
chemical_list: D018797:Cell Cycle Proteins; D009363:Neoplasm Proteins; D020033:Protein Isoforms; D064249:Securin; C578002:pituitary tumor-transforming protein 1, human; C578003:pituitary tumor-transforming protein 2, human; D010450:Endopeptidases; C577987:ESPL1 protein, human; D064247:Separase